arXiv subjects

Aruna Goswami

Publications and source records attributed to Aruna Goswami.

46 records · Page 3Linked to original sources

arXiv2012

The Stellar parametrization using Artificial Neural Network

An update on recent methods for automated stellar parametrization is given. We present preliminary results of the ongoing program for rapid parametrization of field stars using medium resolution spectra obtained using Vainu Bappu Telescope at VBO, Kavalur, India. We have used Artificial Neural Network for estimating temperature, gravity, metallicity and absolute magnitude of the field stars. The network for each parameter is trained independently using a large number of calibrating stars. The trained network is used for estimating atmospheric parameters of unexplored field stars.

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arXiv2010

HE 1015-2050: Discovery of a Hydrogen-deficient carbon star at high galactic latitude

Medium resolution spectral analysis of candidate Faint High Latitude Carbon (FHLC) stars from Hamburg/ESO survey has given us the potential to discover objects of rare types. Two primary spectral characteristics of R Coroanae Borealis (RCB) stars are hydrogen deficiency and weaker CN bands relative to C_{2} bands. They are also characterized by their characteristic location in the J-H, H-K plane with respect to cool carbon stars. From a spectral analysis of a sample of 243 candidate FHLC stars, we have discovered a hydrogen-deficient carbon (HdC) star HE 1015-2050, at high Galactic latitude. A differential analysis of its spectrum with that of the spectrum of U Aquarii (U Aqr), a well-known cool HdC star of RCB type, provides sufficient evidence to put this object in a group same as that of U Aqr. Further, it is shown that HE 1015-2050 does not belong to any of the C-star groups CH, C-R, C-N or C-J. Cool RCB stars form a group of relatively rare astrophysical objects; approximately 51 are known in the Galaxy and some 18 in the Large Magellanic Clouds (LMC) and five in Small Magellanic Cloud (SMC). The present discovery adds a new member to this rare group. Although its spectral characteristics and its location in the J-H vs H-K plane places HE 1015-2050 in the same group to which U Aqr belongs, extended photometric observations would be useful to learn if there is any sudden decline in brightness, this being a characteristic property of HdC stars of RCB type.

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arXiv2010

Evidence of V-band polarimetric separation of carbon stars at high Galactic latitude

Polarization is an important indicator of stellar evolution, especially for stars evolving from red-giant stage to planetary nebulae. However, not much is known about the polarimetric properties of the carbon-enhanced metal-poor (CEMP) stars, although they have been well studied in terms of photometric as well as low- and high-resolution spectroscopy. We report here first-ever estimates of V-band polarimetry of a group of CEMP stars. V-band polarimetry was planned as the V-band is known to show maximum polarization among BVRI polarimetry for any scattering of light caused due to dust. Based on these estimates the program stars show a distinct classification into two: one with p% < 0.4 and the other with p% > 1. Stars with circumstellar material exhibit a certain amount of polarization that may be caused by scattering of starlight due to circumstellar dust distribution into non-spherically symmetric envelopes. The degree of polarization increases with asymmetries present in the geometry of the circumstellar dust distribution. Our results reflect upon these properties. While the sample size is relatively small, the polarimetric separation of the two groups (p% < 0.4 and p% > 1) is very distinct; this finding, therefore, opens up an avenue of exploration with regard to CEMP stars.

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arXiv2010

HD 209621: Abundances of neutron-capture elements

High resolution spectra obtained from the Subaru Telescope High Dispersion Spectrograph have been used to update the stellar atmospheric parameters and metallicity of the star HD 209621. We have derived a metallicity of [Fe/H] = -1.93 for this star, and have found a large enhancement of carbon and of heavy elements, with respect to iron. Updates on the elemental abundances of four s-process elements (Y, Ce, Pr, Nd) along with the first estimates of abundances for a number of other heavy elements (Sr, Zr, Ba, La, Sm, Eu, Er, Pb) are reported. The stellar atmospheric parameters, the effective temperature, Teff, and the surface gravity, log g (4500 K, 2.0), are determined from LTE analysis using model atmospheres. Estimated [Ba/Eu] = +0.35, places the star in the group of CEMP-(r+s) stars; however, the s-elements abundance pattern seen in HD 209621 is characteristic of CH stars; notably, the 2nd-peak s-process elements are more enhanced than the first peak s-process elements. HD 209621 is also found to show a large enhancement of the 3rd-peak s-process element lead (Pb) with [Pb/Fe] = +1.88. The relative contributions of the two neutron-capture processes, r- and s- to the observed abundances are examined using a parametric model based analysis, that hints that the neutron-capture elements in HD 209621 primarily originate in s-process.

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arXiv2009

The CH fraction of Carbon stars at high Galactic latitudes

CH stars form a distinct class of objects with characteristic properties like iron deficiency, enrichment of carbon and overabundance in heavy elements. These properties can provide strong observational constraints for theoretical computation of nucleosynthesis at low-metallicity. An important question is the relative surface density of CH stars which can provide valuable inputs to our understanding on the role of low to intermediate-mass stars in the early Galactic chemical evolution. Spectroscopic characterization provides an effective way of identifying CH stars. The present analysis is aimed at a quantitative assessment of the fraction of CH stars in a sample of stars using a set of spectral classification criteria. The sample consists of 92 objects selected from a collection of candidate Faint High Latitude Carbon stars from the Hamburg/ESO survey. Medium resolution (R ~ 1300) spectra for these objects were obtained using OMR at VBO, Kavalur and HFOSC at HCT, IAO, Hanle, during 2007 - 2009 spanning a wavelength range 3800 - 6800 A. Spectral analysis shows 36 of the 92 objects to be potential CH stars; combined with our earlier studies (Goswami 2005, Goswami et al. 2007) this implies ~ 37% (of 243) objects as the CH fraction. We present spectral descriptions of the newly identified CH star candidates. Estimated effective temperatures, 12C/13C isotopic ratios and their locations on the two colour J-H vs H-K plot are used to support their identification.

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arXiv2006

A high-resolution spectral analysis of three carbon-enhanced metal-poor stars

We present results of an analysis of high-resolution spectra obtained with the Subaru Telescope High Dispersion Spectrograph, of two Carbon-Enhanced Metal-Poor stars, HE 1305+0007 and HE 1152-0355, and of the classical CH star HD 5223. All of these stars have relatively low effective temperatures (4000-4750 K) and high carbon abundances. The stellar atmospheric parameters indicate that they all have surface gravities consistent with a present location on the red-giant branch, and metallicities of [Fe/H] = -2.0 (HE 1305+0007, HD 5223) and [Fe/H]= -1.3 (HE 1152-0355). In addition to their large enhancements of carbon all three stars exhibit strong enhancements of the s-process elements relative to iron. HE 1305+0007 exhibits a large enhancement of the 3rd-peak s-process element lead, with [Pb/Fe] = +2.37, as well as a high abundance of the r-process element europium, [Eu/Fe] = +1.97. Thus, HE 1305+0007 joins the growing class of the so-called ``Lead Stars'', and also the class of objects that exhibit the presence of both r-process and s-process elements, the CEMP-r/s stars. The large enhancement of n-capture elements exhibited by HE 1152-0355 and HD 5223 are more consistent with the abundance patterns generally noticed in CH stars. The elemental abundance distributions observed in these stars are discussed in the light of existing theories of CH star formation, as well as the suggested formation scenarios of the CEMP-r/s group.

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arXiv2005

CH stars at High Galactic Latitudes

In the present work we report on several CH stars identified in a sample of Faint High Latitude Carbon stars from Hamburg survey and discuss their medium resolution spectra covering a wavelength range 4000 - 6800 Å. Estimation of the depths of bands (1,0) $^{12}$C$^{12}$C $λ$4737 and (1,0) $^{12}$C$^{13}$C $λ$4744 in these stars indicate isotopic ratio $^{12}$C/$^{13}$C ${\sim}$ 3, except for a few exceptions; these ratios are consistent with existing theories of CH stars evolution. The stars of Hamburg survey, a total of 403 objects were reported to be carbon star candidates with strong C$_{2}$ and CN molecular bands. In the first phase of observation, we have acquired spectra of ninety one objects. Inspection of the objects spectra show fifty one objects with C$_{2}$ molecular bands in their spectra of which thirteen stars have low flux below about 4300 Å. Twenty five objects show weak or moderate CH and CN bands, twelve objects show weak but detectable CH bands in their spectra and there are three objects which do not show any molecular bands due to C$_{2}$, CN or CH in their spectra. Objects with C$_{2}$ molecular bands and with good signals bluewards of 4300 Åwhich show prominent CH bands in their spectra are potential candidate CH stars. Thirty five such candidates are found in the present sample of ninty one objects observed so far. The set of CH stars identified could be the targets of subsequent observation at high resolution for a detail and comprehensive analysis for understanding their role in early Galactic chemical evolution.

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arXiv2002

Spectroscopic Survey of Field stars: A search for Metal-Poor stars

We have undertaken a spectroscopic survey of field stars to find metal-poor objects among them. Observations are carried out using OMR spectrograph attached to 2.3m telescope at VBO Kavalur, India. The sample of candidate stars are chosen from prismatic survey of Beers and his collaborators covering a large part of the Galaxy. At the first phase of this project, the analysis is completed for a set of 19 relatively hot stars (Teff in 6000 to 8000K range). The metallicities of the program stars are derived by synthesizing the spectrum in the wavelength range 4900 to 5400A for different metallicities and matching them with the observed spectra. More than half of the candidate stars were found to show [Fe/H] in -0.7 to -1.2 range. Two most metal-poor stars have [Fe/H] values of -1.3 and -1.8. It appears that metal-poor candidates suggested by Beers et al. from their prismatic survey has a very significant fraction of metal-poor stars. The significantly metal-poor stars found so far would be studied in detail using high resolution spectra to understand nucleosynthesis processes that might have occurred in early Galaxy.

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arXiv2002

High-resolution spectroscopy of QY Sge -- An obscured RV Tauri variable?

The first high-resolution optical spectra of QY Sge are presented and discussed. Menzies & Whitelock (1988) on the basis of photometry and low-resolution spectra suggested that this G0I supergiant was obscured by dust and seen only by scattered light from a circumstellar reflection nebula. The new spectra confirm and extend this picture. Photospheric lines are unusually broad indicating scattering of photons from dust in the stellar wind. Presence of very broad Na D emission lines is confirmed. Sharp emission lines from low levels of abundant neutral metal atoms are reported for the first time. An abundance analysis of photospheric lines shows that the stellar atmosphere is of approximately solar composition but with highly condensible (e.g., Sc and Ti) elements depleted by factors of 5 to 10.

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arXiv2000

Abundance evolution of intermediate mass elements (C to Zn) in the Milky Way halo and disk

We present a comprehensive study of the evolution of the abundances of intermediate mass elements, from C to Zn, in the Milky Way halo and in the local disk. We use a consistent model to describe the evolution of those two galactic subsystems. The halo and the disk are assumed to evolve independently, both starting with gas of primordial composition, and in different ways: strong outflow is assumed to take place during the $\sim$1 Gyr of the halo formation, while the disk is built by slowly infalling gas. This description of the halo+disk evolution can correctly account for the main observational constraints (at least in the framework of simple models of galactic chemical evolution). We utilise then metallicity dependant yields to study the evolution of all elements from C and Zn. Comparing our results to an extensive body of observational data (including very recent ones), we are able to make a critical analysis of the successes and shortcomings of current yields of massive stars. Finally, we discuss qualitatively some possible ways to interpret the recent data on oxygen vs iron, which suggest that oxygen behaves differently from the other alpha-elements.

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